This invention relates to diesel engines of the type using electrically energized glow plugs for aid in cold engine starting and more particularly of the type including an indicating lamp to signal the initial energization period of the glow plug and thus warn the operator not to attempt starting the engine during such period.
Diesel engines of the type used in motor vehicles are known to require aid in cold starting. This aid is commonly provided in the form of glow plugs which may be electrically energized to heat to a temperature sufficient to help initiate combustion of the fuel and air in the combustion chambers as the engine is cranked by a starting motor. In the case of vehicles with 12 volt electrical systems, a 12 volt rated glow plug may take a considerable length of time to heat to the required temperature, particularly during cold winter weather. Therefore, it has become the practice in at least some engines to provide glow plugs rated for operation at a lower voltage, which glow plugs are energized with the full 12 volts of the electrical system for fast heating. Since such glow plugs typically reach the required temperature in approximately seven seconds or less and may burn out if subjected to energization for significantly longer times, control means are provided for allowing an initial energization period of the required duration and then providing an automatic substantial reduction in current to the glow plug. One type of control system provides full energization for an initial period during which the glow plug heats to the required temperature and then alternately deenergizes and energizes the glow plug in a cyclical manner to maintain the glow plug at the required temperature until the engine is securely started.
It is also customary to provide an indication to the vehicle operator upon initial actuation of the vehicle ignition switch that the engine is not yet ready to start. The typical means of providing such an indication is an indicating lamp on the vehicle dashboard which is energized, upon initial actuation of the ignition switch, concurrently with the glow plugs during their initial energization. Although some control systems energize the indicating lamp concurrently with the glow plugs at all times, this leads, in a system such as that described above wherein the initial glow plug energization period is followed by a cyclical energization, to a cyclical energization of the indicating lamp after the engine is ready to start. Many designers of such systems, however, believe that the indicator lamp should be energized only during the time when the engine is not ready to start and should remain deenergized once the engine becomes ready to start, regardless of the cyclical energization of the glow plugs, so that any further energization of the lamp may be used to signal a fault in the system.
There are some prior art systems which provide for an indicating lamp which is energized only during the initial energization of the glow plugs. A variety of means are provided for accomplishing this goal. One system, for example, provides separate means to energize the glow plugs during the initial and subsequent cyclical energizations, with the initial energization means also causing the energization of the indicator lamp. This accomplishes the purpose, but at the expense of two separate glow plug energization means, which may not be desirable for all systems. Another system, shown in the Sundeen U.S. Pat. No. 4,177,785 , provides a self-latching relay actuated at the end of the initial energization of the glow plugs which breaks the indicator lamp circuit to extinguish the lamp and thus maintains the lamp deenergized through subsequent cyclical energizations of the glow plugs. This approach also works, but at the expense of a separate, self-latching relay. A further approach is shown in the Steele U.S. Pat. No. 4,307,688 . This system provides a transistor shunt current path around an RC timer during the initial energization of the glow plugs and turns off the transistor to remove the shunt path during the first deenergization of said glow plugs. The capacitor of the RC timer thus charges up to block subsequent current flow through the lamp and activate a comparator latch to prevent further activation of the transistor shunt current path during subsequent cyclical energization of the glow plugs. This approach also works but at the expense of an RC timer, Darlington transistor and comparator latch.